High-k dielectric gate structures resistant to oxide growth at the dielectric/silicon substrate interface and methods of manufacture thereof

ABSTRACT

Methods for fabricating gate electrode/high-k dielectric gate structures having an improved resistance to the growth of silicon dioxide (oxide) at the dielectric/silicon-based substrate interface. In an embodiment, a method of forming a transistor gate structure comprises: incorporating nitrogen into a silicon-based substrate proximate a surface of the substrate; depositing a high-k gate dielectric across the silicon-based substrate; and depositing a gate electrode across the high-k dielectric to form the gate structure. In one embodiment, the gate electrode comprises titanium nitride rich in titanium for inhibiting diffusion of oxygen.

BACKGROUND

1. Field

This invention relates to integrated circuits, and particularly tofabricating high-k dielectric gate structures having improved resistanceto the growth of silicon dioxide at the dielectric/silicon-basedsubstrate interface.

2. Description of Background

Integrated circuits often employ active devices known as transistorssuch as field effect transistors (FETs). A metal-oxide-semiconductorfield effect transistor (MOSFET) includes a silicon-based substratecomprising a pair of impurity regions, i.e., source and drain junctions,spaced apart by a channel region. A gate electrode is dielectricallyspaced above the channel region of the silicon-based substrate. Thejunctions can comprise dopants which are opposite in type to the dopantsresiding within the channel region. MOSFETs comprising n-type dopedjunctions are referred to as NFETs, and MOSFETs comprising p-type dopedjunctions are referred to as PFETs. The gate electrode can serve as amask for the channel region during the implantation of dopants into theadjacent source and drain junctions. An interlevel dielectric can bedisposed across the transistors of an integrated circuit to isolate thegate areas and the junctions. Ohmic contacts can be formed through theinterlevel dielectric down to the gate areas and/or junctions to couplethem to overlying interconnect lines.

The gate dielectric interposed between the channel and the gateelectrode of MOSFETs was once primarily made of thermally grown silicondioxide (oxide). Due to the need for integrated circuits having higheroperating frequencies, the thickness of the oxide gate dielectric hassteadily decreased to increase the gate capacitance and hence the drivecurrent of MOSFETs. However, as the thickness of the oxide gatedielectric has decreased, leakage currents through the gate dielectrichave increased, leading to reduced device reliability. As such, theoxide gate dielectric is currently being replaced with dielectricshaving higher dielectric constants (k) than oxide, i.e., k>3.8. Such“high-k dielectrics” provide for increased gate capacitance without thedetrimental effect of leakage current.

MOSFETs that include a metal gate electrode/high-k dielectric stacksuffer from the drawback of experiencing oxide growth at the interfaceof the high-k dielectric and the silicon-based substrate. This oxidegrowth can occur as a result of ambient oxygen and/or oxygen in areas ofthe integrated circuit near the MOSFET diffusing to the high-kdielectric/substrate interface. This oxygen diffusion is more likely tooccur when the integrated circuit is subjected to high temperaturesduring anneal steps and/or thermal steps. The oxide growth canundesirably increase the thickness of the gate dielectric with an oxidehaving a lower k value than that of the original gate dielectric. Themagnitude of the oxide growth is dependent upon the size of the MOSFETand the nature of nearby structures. Unfortunately, the oxide growth canundesirably cause a “width effect” in narrow width (or length) MOSFETsthat is often manifested by non-area scaling of the gate leakage currentin such small area devices. Device characteristics of the width effectinclude a shift in MOSFET threshold voltage (V_(T)) and drive currentdegradation.

SUMMARY

The shortcomings of the prior art are overcome and additional advantagesare provided through the provision of methods for fabricating gateelectrode/high-k dielectric gate structures having an improvedresistance to the growth of silicon dioxide (oxide) at thedielectric/silicon-based substrate interface. In an embodiment, a methodof forming a transistor gate structure comprises: incorporating nitrogeninto a silicon-based substrate proximate a surface of the substrate;depositing a high-k gate dielectric across the silicon-based substrate;and depositing a gate electrode across the high-k dielectric to form thegate structure. In one embodiment, the gate electrode comprises titaniumnitride rich in titanium for inhibiting diffusion of oxygen.

In another embodiment, a transistor gate structure comprises: asilicon-based substrate comprising nitrogen incorporated proximate asurface of the substrate; a high-k gate dielectric disposed upon thesilicon-based substrate; and a gate electrode disposed upon the high-kgate dielectric. In a particular embodiment, the gate electrodecomprises titanium nitride rich in titanium.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIGS. 1-4 illustrate one example of the fabrication of a MOSFETcomprising a high-k dielectric gate structure capable of resisting oxidegrowth at the high-k dielectric/silicon-based substrate interface.

The detailed description explains the preferred embodiments of theinvention, together with advantages and features, by way of example withreference to the drawings.

DETAILED DESCRIPTION

Turning now to the drawings in greater detail, it will be seen thatFIGS. 1-4 illustrate an exemplary embodiment of a method for fabricatinggate electrode/high-k dielectric gate structures that are more resistantto the growth of oxide at the high-k dielectric/silicon-based substrateinterface. As shown in FIG. 1, a bulk substrate 10 comprising singlecrystalline silicon that has been slightly doped with n-type or p-typedopants is first obtained. Alternatively, a semiconductor layer 10 canbe formed upon an insulation layer (not shown) to create asilicon-on-insulator structure. Shallow trench isolation (STI)structures 12 comprising, e.g., oxide, can be formed in thesilicon-based substrate 10 on opposite sides of ensuing MOSFET devicesto isolate different active areas of the substrate 10.

After the formation of the STI structures 12, a nitridation process canbe performed to incorporate nitrogen into the silicon-based substrate 10and the STI structures 12 proximate their surfaces to form interfaciallayer 14. The nitrogen can be introduced at a dosage greater than about5×10¹⁴ atoms/cm² to ensure that the resulting interfacial layer 14,which includes strong silicon-nitrogen (Si—N) bonds, provides adequateprotection against the growth of oxide (SiO₂) upon the substrate 10.Examples of suitable nitridation processes include but are not limitedto exposing the silicon-based substrate 10 and the STI structures 12 toa nitrogen-bearing plasma or subjecting the substrate 10 to a thermalanneal or to rapid thermal processing in the presence of anitrogen-bearing gas. The nitrogen-bearing plasma or gas can comprise,for example, nitric oxide (NO), nitrous oxide (N₂O), ammonia (NH₃),nitrogen gas (N₂), or a combination comprising at least one of theforegoing gases. By way of example, the thermal anneal can be performedat a temperature of about 400° C. to about 1050° C., more specificallyabout 600° C. to about 1000° C., or even more specifically about 700° C.to about 900° C., for a period of about 1 minute to about 60 minutes.Rapid thermal processing (RTP) involves rapidly increasing thetemperature of a substrate to a target temperature where it ismaintained for a relatively short period of time. In this case, thetemperature of the substrate 10 can be increased to a temperatureranging from about 600° C. to about 1200° C., more specifically about850° C. to about 1150° C., or even more specifically about 950° C. toabout 1100° C., where it is maintained for a period of about 1 second toabout 5 minutes.

Turning now to FIG. 2, a high-k gate dielectric 16 can be depositedacross the interfacial layer 14 by, e.g., sputter deposition, atomiclayer deposition, or chemical vapor deposition. As used herein, the term“high-k dielectric” refers to a dielectric having a dielectric constant,k, greater than about 4.0, which is higher than the k value of oxide.Examples of suitable high-k dielectric materials include but are notlimited to hafnium oxide (HfO₃), hafnium silicon oxynitride (HfSiON),tantalum oxide (Ta₂O₅), aluminum oxide (Al₂O₃), and combinationscomprising at least one of the foregoing dielectrics. Without intendingto be limited by theory, it is believed that a Si—N—O network forms inthe interfacial layer 14 as a result of performing the nitridationprocess followed by the deposition of the high-k gate dielectric 16. Itis also believed that the Si—O—N network can block the diffusion ofoxygen to the silicon-based substrate 10 and thereby reduce the amountof area dependent growth of oxide at the substrate surface. Further, itis believed that the strong Si—N bonds present in this network cancontribute to the ability of the interfacial layer 14 to prevent oxygenfrom migrating through layer 14 and to form a relatively stable layer onthe silicon-based substrate 10.

Next, as shown in FIG. 3, a conductive gate electrode material 18 can bedeposited across the high-k gate dielectric 16. The gate electrodematerial 18 is desirably optimized to reduce its oxygen content and thediffusion of oxygen through the material 18. In one embodiment, the gateelectrode material 18 is titanium nitride (TiN) rich in titanium (Ti).As used herein, the term “rich in titanium” refers to a material havinga greater quantity of titanium than any other component. The depositionof the TiN rich in Ti can be performed using, e.g., physical vapordeposition, atomic layer deposition, chemical vapor deposition, ormolecular beam epitaxy. It is desirable to form the TiN rich in Ti byphysical vapor deposition using a reduced partial pressure of N₂ forreaction with Ti relative to an inert gas such as argon (Ar). Forexample, the Ar/N₂ mole ratio can be greater than or equal to about25/20. In alternative embodiments, the gate electrode material 18 couldbe formed from other conductive materials rich in the metal componentsuch as tantalum nitride (TaN) rich in tantalum or tantalum carbide(TaC) rich in tantalum.

Subsequently, the gate electrode material 18 can be patterned usinglithography and an anisotropic etch technique, e.g., reactive ionetching, to define sidewall surfaces of the gate electrode 18, as shownin FIG. 4. Although not shown, dielectric spacers can then be formedupon the sidewall surface of the gate electrode 18 via chemical vapordeposition of a dielectric followed by an anisotropic etch technique,which etches the dielectric at a faster rate in the vertical directionthan in the horizontal direction. Without intending to be limited bytheory, it is believed that the presence of the gate electrode 16comprising TiN rich in Ti in the gate structure affords additionalprotection against the growth of oxide at the surface of thesilicon-based substrate 10 because the gate electrode 16 has arelatively low oxygen content and forms a good barrier to the diffusionof oxygen.

FIG. 4 also depicts the formation of source and drain (S/D) junctions 20in the silicon-based substrate 10 on opposite sides of the gateelectrode 18. In the case where an NFET is being formed, the S/Djunctions 20 can be formed through the implantation of p-type dopantstherein. In the case wherein a PFET is being formed, the S/D junctions20 can be formed through the implantation of n-type dopants therein.While FIG. 4 only depicts the formation of one type of FET, it isunderstood that both NFETs and PFETs can be formed upon thesilicon-based substrate 10 when it is desirable to form a complementarymetal-oxide-semiconductor (CMOS) integrated circuit. During theimplantation step, the gate electrode 18 and the dielectric sidewallspacers, if present, act as a mask that inhibits dopants from passinginto an underlying channel region of the substrate 10. Examples ofn-type dopants include but are not limited to arsenic, phosphorus, andcombinations comprising at least one of the foregoing dopants. Examplesof p-type dopants include but are not limited to boron, borondifluoride, and combinations comprising at least one of the foregoingdopants. The dopants can be subsequently activated via a thermal annealprocess.

The MOSFET formed in FIG. 4 includes a stacked gate structure comprisinga nitrogen-incorporated interfacial layer 14 disposed upon thesilicon-based substrate 10, a high-k gate dielectric 16 disposed uponthe interfacial layer 14, and a gate electrode 18 disposed upon thehigh-k gate dielectric 16. The gate structure can have a narrow gatewidth of, for example, about 0.1 micrometer to about 50 micrometers. Theinterfacial layer 14 can have a thickness of about 0.5 nanometers (nm)to about 2 nm, more specifically about 0.5 nm to about 1 nm. The high-kgate dielectric 16 can have a thickness of about 1 nm to about 5 nm.Moreover, if the gate electrode 18 is a Ti-rich TiN, it can have athickness of about 1 nm to about 20 nm.

As described above, the gate structure shown in FIG. 4 includes both aninterfacial layer 14 comprising Si—N—O between the substrate 10 and thehigh-k gate dielectric 16 and an optimized gate electrode 18 (e.g., aTi-rich TiN gate electrode) for improving the resistance of the gatestructure to the growth of oxide upon the substrate surface.Consequently, the MOSFET comprising this gate structure is less likelyto exhibit characteristics related to width effect such as a shift inV_(T) and a decrease in drive current.

As used herein, the terms “a” and “an” do not denote a limitation ofquantity but rather denote the presence of at least one of thereferenced items. Moreover, ranges directed to the same component orproperty are inclusive of the endpoints given for those ranges (e.g.,“about 5 nm to about 20 nm,” is inclusive of the endpoints and allintermediate values of the range of about 5 nm to about 20 nm).Reference throughout the specification to “one embodiment”, “anotherembodiment”, “an embodiment”, and so forth means that a particularelement (e.g., feature, structure, and/or characteristic) described inconnection with the embodiment is included in at least one embodimentdescribed herein, and might or might not be present in otherembodiments. In addition, it is to be understood that the describedelements may be combined in any suitable manner in the variousembodiments. Unless defined otherwise, technical and scientific termsused herein have the same meaning as is commonly understood by one ofskill in the art to which this invention belongs.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A method of forming a transistor gate structure, the methodcomprising: incorporating nitrogen into a silicon-based substrateproximate a surface of the substrate; depositing a high-k gatedielectric across the silicon-based substrate; and depositing a gateelectrode across the high-k dielectric to form the gate structure. 2.The method of claim 1, wherein the nitrogen is incorporated into thesilicon-based substrate proximate the surface of the substrate at adosage greater than about 5×10¹⁴ atoms/cm².
 3. The method of claim 1,wherein said incorporating the nitrogen into the silicon-based substratecomprises exposing the silicon-based substrate to a nitrogen-bearingplasma.
 4. The method of claim 3, wherein the nitrogen-bearing plasmacomprises nitric oxide, nitrous oxide, ammonia, nitrogen gas, or acombination comprising at least one of the foregoing gases.
 5. Themethod of claim 1, wherein said incorporating nitrogen into thesilicon-based substrate comprises annealing or rapid thermal processingthe substrate in the presence of a nitrogen-bearing gas.
 6. The methodof claim 5, wherein the nitrogen-bearing gas comprises nitric oxide,nitrous oxide, ammonia, nitrogen gas, or a combination comprising atleast one of the foregoing gases.
 7. The method of claim 1, wherein thegate electrode comprises titanium nitride rich in titanium, tantalumnitride rich in tantalum, or tantalum carbide rich in tantalum forinhibiting diffusion of oxygen.
 8. The method of claim 7, wherein saiddepositing the gate electrode comprises is performed by physical vapordeposition, atomic layer deposition, chemical vapor deposition, ormolecular beam epitaxy.
 9. The method of claim 1, wherein saiddepositing the gate electrode comprises physical vapor deposition oftitanium nitride rich in titanium using a gas comprising argon and N₂present at a mole ratio of greater than or equal to about 25/20.
 10. Themethod of claim 1, wherein the high-k gate dielectric comprises hafniumoxide, hafnium silicon oxide, tantalum oxide, aluminum oxide, or acombination comprising at least one of the foregoing dielectrics. 11.The method of claim 1, wherein the gate structure comprises a Si—N—Olayer at an interface between the silicon-based substrate and the high-kgate dielectric.
 12. A transistor gate structure, comprising: asilicon-based substrate comprising nitrogen incorporated proximate asurface of the substrate; a high-k gate dielectric disposed upon thesilicon-based substrate; and a gate electrode disposed upon the high-kgate dielectric.
 13. The transistor gate structure of claim 12, whereinthe nitrogen is present in the silicon-based substrate at a dosage ofgreater than about 5×10¹⁴ atoms/cm².
 14. The transistor gate structureof claim 12, wherein the gate electrode comprises titanium nitride richin titanium, tantalum nitride rich in tantalum, or tantalum carbide richin tantalum for inhibiting diffusion of oxygen to underlying layers. 15.The transistor gate structure of claim 12, wherein the high-k gatedielectric comprises hafnium oxide, hafnium silicon oxide, tantalumoxide, aluminum oxide, or a combination comprising at least one of theforegoing dielectrics.
 16. The transistor gate structure of claim 12,wherein the gate structure comprises a Si—N—O layer at an interfacebetween the silicon-based substrate and the high-k gate dielectric forinhibiting the diffusion of oxygen to the substrate.
 17. The transistorgate structure of claim 16, wherein the Si—N—O layer has a thickness ofabout 0.5 nanometer to about 2 nanometers.
 18. The transistor gatestructure of claim 12, wherein the high-k gate dielectric has athickness of about 1 nanometer to about 5 nanometers.
 19. The transistorgate structure of claim 14, wherein the gate electrode has a thicknessof about 1 nanometer to about 20 nanometers.
 20. The transistor gatestructure of claim 14, wherein the gate structure has a width of about0.1 micrometer to about 50 micrometers.